Studies will continue on the development of structural analogs of the neurohypophysial hormones that will act as more effective and specific antagonists. Particular efforts will be made to find an effective in vivo antagonist of the antidiuretic action of these hormones. We will use presently available antagonists of the vasopressor, oxytocic and milk-ejection responses as probes to study the actions of endogenous neurohypophysial peptides under physiological and pathological circumstances. We will also explore further the influence of other hypothalamic peptides, particularly endorphins and substance P, on the release of neurohypophysial hormones.